Historical seismicity is the historical records of earthquakes preserved in different form such as written history, chronicles, inscription etc which plays an important role in the seismic hazard assessment because instrumentally recorded earthquakes are lacking before the current century. Historical events must be available for a long period of human civilization which should throw light on the extent of damage besides the date and place of occurrence.
The earthquake of 1255 A. D. has been reported to destroy many houses and temples and killing one third to one fourth population of the Kathmandu Valley. The assigned intensity is about X in MM scale (Chitrakar and Pandey, 1986). The earthquake of 1408 A. D. has been reported to destroy the Machhendra Nath temple of Patan. Similarly the earthquake of 1681 A. D. and 1810 A. D. have been reported to occur but the exact location of these earthquakes are not known.
Recent research on historical data has well constrained on the source size, magnitude and possible location of 1833 A. D. event (R.Bilham,1995) which devastated Kathmandu valley. Its magnitude is reported to be of Mb=7.8 with possible rupture length of more than 70 km and the event is located at 50 km North - North East of Kathmandu.
The human casualties are reported to be less than 500, which may be due to occurrence of two large foreshocks. The earthquake of 1934 A. D. is the most devastating earthquake ever occurred in the territory of Nepal with casualties of more than 16000 people including from Nepal and India put together. The rupture length is estimated to be 200 Km 100 Km (Molnar and Pandey, 1994).
The great earthquake, which occurred in Nepal, was Bihar- Nepal earthquake of 1934 A. D. Assam great earthquake of 1897, Kangra earthquake 1905, and Assam earthquake 1950 were felt in Nepal. The earthquake of 1833 also affected the Kathmandu Valley. The record of historical earthquake is not complete which poses a problem in assessing the recurrence period of great earthquakes. From the available data there has been no great earthquakes of magnitude >8.0 in the gap between the earthquakes of 1905 A. D and 1934 A. D. and there is a real threat that a major earthquake may occur in this gap that will affect Western Nepal.